Add google error-prone in compile step. (#223)

* Add google error-prone in compile step.
  Note that FallThrough violations have been reduced to warnings. See issue #222
* Enable error-prone in the java 8 profile only, because the plugin was compiled for java 8, and cannot be used in openjdk7.

pom.xml

@@ -235,13 +235,41 @@
   </build>
   <profiles>
     <profile>
-      <!-- Profile for Java 8: Disable Doclint -->
+      <!-- Profile for Java 8: Enable error-prone, Disable Doclint -->
       <id>java8</id>
       <activation>
         <jdk>[1.8,)</jdk>
       </activation>
       <build>
         <plugins>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-compiler-plugin</artifactId>
+            <version>3.3</version>
+            <configuration>
+              <compilerId>javac-with-errorprone</compilerId>
+              <forceJavacCompilerUse>true</forceJavacCompilerUse>
+              <compilerArgs>
+                <arg>-Xep:FallThrough:WARN</arg>
+              </compilerArgs>
+              <source>7</source>
+              <target>7</target>
+            </configuration>
+            <dependencies>
+              <dependency>
+                <groupId>org.codehaus.plexus</groupId>
+                <artifactId>plexus-compiler-javac-errorprone</artifactId>
+                <version>2.8</version>
+              </dependency>
+              <!-- override plexus-compiler-javac-errorprone's dependency on
+                   Error Prone with the latest version -->
+              <dependency>
+                <groupId>com.google.errorprone</groupId>
+                <artifactId>error_prone_core</artifactId>
+                <version>2.1.1</version>
+              </dependency>
+            </dependencies>
+          </plugin>
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-javadoc-plugin</artifactId>